The present invention relates to a Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 -based dielectric ceramic composition for use in a microwave range and a process for preparing the same. More particularly, the present invention is concerned with a dielectric ceramic composition wherein the regulation of the temperature coefficient at the resonance frequency has been made possible with high reproducibility while maintaining a high Q value by making the barium composition ratio slightly short of the stoichiometric amount, and a process for preparing a dielectric ceramic composition wherein a calcination product is sintered without wet pulverization.
This dielectric ceramic composition can be utilized in, e.g., a dielectric resonator, a dielectric substrate for MIC, etc. for use in a high-frequency range such as a microwave or millimeter wave region.
As regards Ba(Zn, Ta, Zr)O.sub.3 -based material, several researches have been already made. For example, a dielectric ceramic composition for use in a high-frequency range represented by the following general formula is known in the art:
Ba(Zr.sub.x Zn.sub.y Ta.sub.z)O.sub.7/2-x/2-3y/2 wherein 0.02.ltoreq.x.ltoreq.0.13, 0.28.ltoreq.y.ltoreq.0.33, and 0.59.ltoreq.z.ltoreq.0.65, provided that x+y+z=1.
In this composition, when the value of x is changed from 0.02 to 0.13, the temperature coefficient, .tau.f, at the resonance frequency is -1 to 16 ppm/.degree. C. Further, in this composition, the Q value exceeds 10,000 when 0.04.ltoreq.x.ltoreq.0.09.
Further it was reported that the temperature coefficient of the above-described composition at the resonance frequency could be set at an arbitrary value around 0 ppm/.degree. C. by substituting Ni and Co for 70 atomic % or less of Zn and adding 0.1 to 10.0 mole % of a lanthanide oxide.
Various series of material have been developed as a dielectric ceramic composition used in a microwave range, and a Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 -based composition is one of such materials. In general, not only these materials but also other ceramic materials are prepared by the following process. At the outset, raw materials are weighed so as to have a predetermined composition, and they are mixed with each other. The mixture is calcined, wet pulverized, and dried. Subsequently, the dried powder is kneaded with a binder, granulated, molded, and sintered under suitable conditions.
A pure Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 composition free from any additive (stoichiometric composition) is poor in sinterability even when it is sintered at a temperature as high as 1600.degree. to 1700.degree. C. Further, in this case, when the sintering is conducted for a long period of time, there occurs abnormal grain growth, which brings about a remarkable lowering in the density, so that it is difficult to control the dielectric constant, .epsilon.r.
However, it was reported that an unloaded Q value at 12 GHz could be improved up to about 14000 by sintering at a temperature of 1350.degree. to 1600.degree. C. for more than ten hours to over one hundred hours [see "Erekutoroniku Seramikkusu (Electronic Ceramic)": Gakkensha K.K., Mar. 1986, pp. 41-45]. In this case, the dielectric constant was about 29 to 30. However, it was also reported that the hot pressing could improve the dielectric constant to about 30.2. The temperature coefficient at the resonance frequency was about 0.+-.0.5 ppm/.degree. C. in this case.
The dielectric constant and the temperature coefficient at resonance frequency can be regulated to some extent by accurately weighing an additive such as Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3, BaZrO.sub.3 or Ba(Ni.sub.1/3 Ta.sub.2/3)O.sub.3 and adding it to the composition.
Preferably, the material used in a microwave dielectric resonator is one which can be endowed with an arbitrary temperature coefficient, .tau.f, with high reproducibility while maintaining a high Q value. Further, it is important that the kind of materials to be used be as few as possible and the material can be easily prepared.
In general, in the production of a dielectric ceramic composition, it is necessary to conduct wet pulverization of a calcination product for the purpose of obtaining a dense and uniform sintering product. However, in the step of wet pulverization followed by the step of drying, variation in the composition occurs due to the loss of some of the raw materials in the apparatus, so that the dielectric characteristics fluctuate and the reproducibility is poor.